In certain applications, owing to the technological limitations of analog-digital coding circuits, digital receivers use sampling frequencies that are much lower than the total reception band, for example a sampling frequency of the order of a GHz for a total band to be processed included in a frequency band of the order of about ten Ghz.
Under these conditions the spectral analysis, performed by digital filtering after signal sampling and coding, gives an ambiguous measurement of the frequency of the signal, owing to spectral aliasings.
To process the spectral aliasings resulting from sub-sampling, these receivers use several reception pathways operating at different sampling frequencies. It is then possible to remove the ambiguity in the frequency measurement on the basis of the various frequency measurements obtained on each of the reception pathways, on the condition that the sampling frequencies are chosen judiciously and that the signal-to-noise ratio is sufficient to allow detection of the signal in each reception pathway. Such a device is in particular described in document EP 1 618 407 A1.
In the cases where the signal-to-noise ratio at the output of the filtering is insufficient, the detection and a fortiori the estimation of the frequency of the signal become impossible. Such is the case for example when the signal to be detected results from a continuous or quasi-continuous emission with low peak power.
A problem to be solved is in particular the detection or the measurement of frequency of a signal with long pulse duration and of low peak power on the basis of a digital receiver having broad frequency band and multiple sub-samplings.